1. Field of the Invention
The present invention relates to a method for determining amounts of UCR (under color removal) to be used for converting a 3-color signal of yellow (Y), magenta (M) and cyan (C) to a 4-color signal of Y, M, C and black (K) (3/4 conversion), and also to an image processing apparatus using said method for determining UCR amounts.
2. Description of the Related Art
In an image processing apparatus for preparing dot film for printing, a 3-color signal separated by a scanner into red (R), green (G) and blue (B) is converted into the 3 colors Y, M and C and these 3 colors are further converted into the 4 colors Y, M, C and K. This is called UCR (under color removal) processing. In UCR processing, the density data for K is prepared from density data the inputted 3 colors of Y, M and C, and a UCR amount is subtracted from the density data the inputted 3 colors of Y, M, and C.
UCR is not the same value for Y, M or C, and it is expressed as a function of the density of generated K and inputted density of Y, M and C for each color. For example, if it is supposed that the density of the generated K is d.sub.K and the density of the inputted C is d.sub.C, UCR relative to C is expressed as a function of d.sub.K and d.sub.C : g (d.sub.C, d.sub.K). For Y and M, it is expressed similarly as a function of the density of K respectively.
The amount of UCR is experimentally obtained and it is set as a fixed value in an image processing apparatus. However, users often want to determine the UCR amount according to the desired printing conditions, and a conventional image processing apparatus cannot satisfy such requirements. For this reason, if a user wants to express UCR characteristics as desired in a conventional image processing apparatus, it is necessary to finely adjust parameters for gradation adjustment and halftone percentage and to totally adjust input/output characteristics of density data by repeated trial and error. This requires tremendous labor and time until satisfactory settings can be reached, and the working efficiency is very low.